The things to look at for a revolver will be very different from an auto. If it is an auto and you know how to take it apart, do so if they will let you. If they will not let you disassemble to check internals...walk away. With a revolver check for proper lock up. Pull the hammer back. Check for movement in the cylinder. Pull the trigger. Does the hammer fall free? Release the trigger. Does the hammer back away from the frame like it should? Now pull the hammer back again. Hold the hammer while pulling the trigger. Lower the hammer a little bit and release the trigger. Watch the transfer bar as you lower the hammer with the trigger released. The transfer bar should go down so the hammer does not strike it. Does it do that? Take an unsharpened pencil. Pull the hammer back. The cylinder should be locked up. Put the pencil in the barrel. Feel the edge of the barrel and cylinder where they meet. Are they lined up properly? If not you could have a timing problem. Walk away.
There are a lot of things to check. Always check the proper operation of all safeties. If a pawn shop will not let you do these checks...walk away.
Romans 1:16 I am not ashamed of the gospel, because it is the power of God for the salvation of everyone who believes: first for the Jew, then for the Gentile.